Volume control for radioreceivers



Aug. 23, 1932. WHEELER 1,873 236 VOLUIE CONTROL FOR RADIOREGEIVERS FiledSept. 22. 1928 2 Sheets-Sheet 1 ATTORNEYS 23, 1932- H. A. WHEELER IVOLUIE CONTROL FOR 'RADIO RECEIVERS 2 Sheets-Sheet 2 Filed Sept. 22.1928 Patented Aug. 23, 1932 UNITED STATES PATENT OFFICE HAROLD LWHEELER, OF JACKSON HEIGHTS, NEW YORK, ASSIGNOR TO HAZELTINECORPORATION, OF JERSEY CITY, NEW JERSEY, A CORPORATION OF DELAWAREVOLUME CONTROL FOR BADIOBECEIVERS Application filed September 22, 1928.Serial No. 307,688.

use in connection with radio receiving equipment with alternatingcurrent vacuum tubes-that is, tubes in which the cathodes are heated byalternating current.

-The ordinary method of regulating the output'volume of a radio receiverconsists in adjustin the cathode heating current of the radiorequencyamplifier tubes. For

reasons to be presently stated, this practice is satisfactory withdirect-current tubes, but not with alternating-current tubes. There aretwo varieties of alternating-current tubes: (1 those in which thecathode is simply a lament of large diameter designed to operate at avery low voltage, and characterized by high inertia with respecttotemperature variations; and 2) those in which the cathode isindirectly heated by means of an auxiliary heating filament., Thevfirstmentioned type of tube is characterized in that the filamenttemperature is quite critical i. e., any material deviation from opti-'mum filament current being likely-to result in an alternating-currenthum. In the second type of tube the cathode temperature variationsfollow the heater current changes too slowly to permit of satisfactoryvolume control through the medium of heater current regulation. Hence itfollows that heating-current regulation is not satisfactory for eitherof the two varieties of: alternatingcurrent tubes.

The invention herein described embraces certain novel arrangements foreffecting volume control by varying the coupling between the receivingantenna and the first radio-frequency amplifying tube or coupling tube.in

this way the effectiveness with which the incoming signal voltage istransmitted from the antenna to the radio-frequency amplifier can beregulated and, in consequence, the output volume controlled at will.face,

this is the simple and obvious procedure in view of the aforementionedcharacteristics of alternating-current tubes, it nevertheless cannot bedone satisfactorily under practical conditions through the agency'ofordinary couplmg devices such as variable inductance couplers orvariable condensers. The reason for this is that such devices will notpermit of a suflicient reduction in coupling when the incoming Waves arevery strong. An ordinary adjustable air condenser can be used as avariable coupling medium for volume control purposes, but such acondenser cannot be made with a small enough minimum capacity to reducestrong signal waves to the required extent. In the case of an ordinaryinductive coupler in which one of the coils is rotatable or otherwiseadjustable with respect. to the other, the-inductive coupling can bereduced substantially to zero, but the zero position is too critical,and there remains the capacitive coupling between the coils. The latteris usually excessive from the volume control standpoint when theincoming signal waves are very strong.

The solution of the problem, then resolves itself into a matter ofdevising ways and means of reducing the inductive or capacitivecoupling, or both, to a very small value without having to utilize anexcessive amount of space. Moreover, it is essential that thearrangement or device employed for this purposebe capable of providing asufficient degree of coupling when the incoming waves are very weak.

The present invention provides for the attainment of these ends andobjects through I ing illustrated in the accompanying drawings anddescribed in detail hereinafter.

In the drawings Fig. 1 is a fragmentary circuit diagrani representingthe input end of a radio receiver. In this figure there are shown anaperiodic antenna circuit and a radio frequency coupling tube, togetherwith a variable ca acitive coupling device in accordance with t e present invention.

Fig. 2 is also a fragmentary circuit diagram of the input end of a radioreceiver. This figure illustrates an antenna circuit and a coupling tubecoupled together with an inductive coupler provided with an ad'ustableshielding device in accordance with t e present invention.

Fig. 3 is a detail view illustrating the manner in which adjustableinductive shielding in accordance with Fig. 2 may be carried out.

Figs. 4 and 5 are sectional views of a capacitive coupling devicesuitable for use as a component part of Fig. 1. Fig. 4 is taken alongthe line 4-4 of Fig. 5 and Fig. 5 is taken along the line 55 of Fig. 4.

Figs. 1 and 2, respectively, are intended to represent, merely by way ofexample, two general and alternate embodiments of the invention, both ofwhich may assume numerous specific forms.

In Fig. 1 the aerial 1 is coupled to the grid 3 of couplingv tube 2through the medium of a direct capacity 4. The grid 3 may be providedwith a leakage path to cathode comprising an inductance 5, or highresistance 6, or both as shown. It is to be ob: served that the cathodeof the coupling tube in Fig. l (as well as in Fig. 2) is indicated asbeing heated by a battery. This showing is intended to be symbolicmerely of a source of heating current; for as already pointed out, thepresent invention is especially useful when employed in connection withsystems wherein the cathodes are heated by alternating current. Themagnitude of capacity 4 should be adequate to provide a sulficiently lowimpedance path to the weakest signal waves of the lowest frequency dealtwith. As shown in Fig. 1, this direct capacity may exist between twoopposed plates 7 and 8, respectively. In actual practice there areusually more than two plates, as will be pointed out in connection withthe description of Figs. 4 and 5. Assuming that the maximum directcapacity between plates 7 and 8 is sufficient to provide an adequatelylow impedance path for the weakest signal wave of lowest frequency dealtwith, it would usually be impossible to sufliciently reduce thiscapacity by merely separating the plates as in the ordinary adjustablecondenser designs. According to the present invention the same result asmight be accomplished by extreme separation of plates 7 and 8 isaccomplished within a very small space through the use of adjustableshielding plates 9. The shielding plates are interleaved with and areadjustable with respect to plates 7 and 8, and they are grounded to themetal shielding 11 in this specification is intended to have thissignificance. When the shieldin plates 9 are completely interleaved withpl ates7 and 8 the coupling between the aerial 1 and grid 3 through thecapacity 4 is reduced to a value closely approaching zero. A specificdevice comprising the capacity coupling and shielding elements of Fig. 1is illustrated in Figs. 4 and 5, and will be described later.

In order to obtain the maximum reduction of coupling the radio-frequencyamplifier circuits should preferably be well shielded from the antennacircuits by means of suitable sheet-1netal shielding, as indicated at 11in Fig. 1. This may consist of the ordinary compartment type ofshielding or any type which will serve to sufiiciently isolate theradio-frequency stages from the antenna. In addition to shielding theamplifier it is highly desirable that the coupling device comprising theplates 7, 8 and 9 be enclosed within a metal shielding box orcompartment as shown in Fig. 1. This compartment is designated byreference numeral 12. The shielding of the coupling device, asillustrated, is effective in the prevention of incidental couplingbetween the antenna and the con denser plate 8 connected with the grid 3of the coupling tube. All the common terminals are shown connected tothe shielding 11 which in turn is grounded at 13. When the arrangementdescribed is designed to deal with exceedingly strong incoming waves theauxiliary shielding provisions, as shown, are especially necessary.

In Fig. 2 an arrangement employing predominantly inductive coupling isillustrated. The coupler comprises two' parallel pancake type coils ofwhich the primary 14 is in series with the aerial 15 and ground 16. Oneterminal of the secondary winding 17 is connected to the grid 18 of thecoupling tube 19 while the other terminal is connected to the metalshielding 20 which is grounded at 21.

An arrangement somewhat similar to that of Fig. 1 is provided forregulating the effective coupling bet-ween coils 14 and 17. Thiscomprises a set of adjustable, grounded shielding plates 22 interleavedwith the coils. Here,.again, as in Fig. 1, the term grounded as appliedto the shielding plate 22 is intended to signify that the shield isconnected to a point of substantially zero highfrequency potential withrespect to the grid 18 of the tube 19; for example, the cathode thereof.When such connection is made the plates 22 comprise a capacitive as wellas an inductive shield. When completely interleaved with the coils theplates 22 reduce the coupling, both inductive and capacitive, tosubstantially zero. The coupling tube and succeeding amplifier areenclosed within a shielding compartment '20, but no shieldmgcorresponding to the compartment 12 of Fig. 1 is indicated in Fig. 2.Whether such additional shielding should be provided depends upon theparticular requirements in any given case.

Fig. 3 is an end view of an'arrangement applicable to the system of Fig.2.- The shielding plates 22 are pivoted on an axis 23, and a suitableknob, not shown, is provided for rotating the shielding plates 22 abouttheir axis. The dot-dash line 24 indicates the position of the shieldingplates when the two coupling coils 14 and 17 are completely shieldedfrom each other. The plates 22 function to regulate simultaneously bothinductive and capacitive coupling between the coils. In the full-lineposition of the shielding plates the maximum coupling is obtained. Itfollows that any desired degree of coupling can be attained by rotatingthe shielding plates.

The arrangement of Figs. 2 and 3 may be utilized to vary the couplingbetween any two coils. In the circuit of Fig. 2, however, it issuggested that the two coils 14 and 17 may be the primary and secondaryof a stepup transformer for transferring the signal from the antennacircuit to the grid 18. This transformer may comprise two coils of theso-called pancake construction mounted in a parallel coaxial positionand approximately resonant with antenna and tube capacities to allfrequencies within a given bandfor example, the broadcast frequency band(from 550 to 1450 kilocycles.)

In Figs. 4 and 5 there is illustrated an adjustable coupling condensersuitable for use in an arrangement such as that of Fig. 1. Fig. 4 is asectional view taken along the line 44 of Fig. 5 while the later figureis a cross sectional view taken axially along the line 5-5 of Fig. 4.This device comprises a metal box consisting of end pieces 25 and 26 anda cylindrical wall 27 supported by the end pieces. The plates 7 and 8 ofFig. 1 are respectively represented in the structure of Figs. 4 and 5 byalternate sets of plates. One such set is designated 28, while the otheris designated 29. These plates are supported on rods 30 and 31,respectively, which in turn are supported by insulating bushings in theend pieces 25 and 26. The plates 28 and 29 are properly spaced bysuitable metal separators, such as 32, and are interleaved as shown. Thenumber of plates employed and their dimensions are determined by themaximum coupling capacity required. The shielding plates correspond ingto 9 in Fig. '1, and designated by refera knob 35 for rotating the shaft34 and plates 33. These latter plates are adj ustably interleaved withthe plates 28 and 29 which are thereby shielded from each other,electrostatically, to any desired degree. In order to reduce as much aspossible the minimum unshielded capacity between the coupling plates 28and .29, they are provided with staggered mounting lugs 36, 37, 38 and39 as shown in Fig. 4. The lugs 36 and 38 are integral parts of the sameand corresponding plates, while the alternate lu 37 and 39 are integralparts of the oppos te plates. As a result of their wide separation andshielding, the capacity betwen the mounting lugs is restricted to a verysmall value. It will be noted that the rotary plates 33 have aconsiderably larger radius than that of the stationary plates 28 and 29and, accordingly, with the former'are interleaved with the latter theyoverlap radially, thereby cutting down residual coupling capacity to assmall a value as may be required.

An additional advantage of the herein described methods of controllingvolume by adjustable shielding, is that the rate of change of volumewhile moving the shielding plates can be controlled, when the set isdesigned, by the shape of the shielding plates. For'ex ample, the-platesmay be made of such a shape that the signal will be reduced one-half foreach 10 rotation on the volume control dial, giving a signal reductionin the ratio of approximately 1:1000 for the last 100 degrees of a 180scale.

Another advantage over prior devices for volume control is that thedegree of shielding varies gradually and continuously during.HClMIStIIleIlt, while the usual wire wound resistance volume controldevice varies in small steps discontinuously, thereby causing clickingnoises in the loud speaker. It should be apparent that the invention issusceptible of embodiment in a variety of forms, and should not beconstrued as limited to the particular arrangements and deviceshereinbefore described. For example, the capacitive coupling device ofFigs. 4 and 5 is applicable to certain forms of tuned antenna circuitsas well as to the untun-ed or a periodic antenna circuit of Fig. 1.Also, any of these devices are applicable to secure adjustable couplingbetween two circuits of any nature for alternating currents of anyfrequency range.

WVhat is claimed is:

A radio receiving system including an antenna circuit, a vacuum tubecircuit electrically shielded therefrom, a capacitive coupling devicehaving three cooperative elements enclosed in a metallic casing, two ofsaid elements being connected in the two said circuits, respectively,and the third element being connected to said casing and adjustablyHAROLD A. WHEELER.

